Autocrypt is a set of guidelines for convenient end-to-end-encryption of e-mails. Developers are working on bringing it to several different e-mail programs.
Delta Chat is a modern messenger. It is like email in a new dress. Just better, safer and user-optimised.
K-9 Mail is an open source email client focused on making it easy to chew through large volumes of email
Enigmail is a seamlessly integrated security add-on for Mozilla Thunderbird. It allows you to use OpenPGP to encrypt and digitally sign your emails and to decrypt and verify messages you receive.